Revising the Gubler-Thomas Model for Powdery Mildew – Adding a Revised High Temperature Threshold, Varietal Susceptibility, and Expanding Public Access

The Gubler Thomas (GT) risk assessment model is a powerful tool that has been used to successfully control grape powdery mildew throughout California, as well as many other grape growing regions of the world. Proper use of the model has significantly reduced the number of fungicide applications significantly in most years while maintaining thorough disease control. Powdery mildew, caused by the fungus Erysiphe necator, can quickly spread throughout a vineyard. Controlling and reducing powdery mildew in grape crops can lead to higher yields and quality in subsequent grapes and wine. In California, powdery mildew germination, growth and reproduction is largely dependent on temperature. If the temperature remains above the pathogens optimal range for too long, the pathogens growth is inhibited. In previous years, our lab has shown that the pathogen can survive higher temperatures than previously thought (Backup, 2009). This led us to begin field trials testing whether a new high temperature threshold could be successfully incorporated into the model, with the goal of reducing fungicide sprays and improving disease control. We report here our finding on our second year of field trials and our continuing efforts in the laboratory to better understand the effects of heat, including a continuing study of the effects of multiple consecutive exposures to lethal and sub-lethal temperatures. In the last two years, we created two field trials that integrated our controlled environment laboratory studies with a vineyard and fungicide application regiment. These field sites were developed to test alternate high temperature thresholds for the GT model. In response to last years data, we have increased the duration that the pathogen must be exposed to 34?C, the lowest high temperature threshold. In addition, we have also included this year a new high temperature threshold modified by a cooperator in Oregon. All of the experimental high temperature models showed significant control in one of our vineyards, although none of the new models were significantly different from the original GT model. Our other vineyard had significantly greater disease pressure. All of the experimental models performed just as well in reducing disease incidence and severity compared to the GT original model. In the 2010 growing season, the unusual mild weather at our field sites made it difficult to test alterations to the high temperature threshold. The alternate models did not differ from one another in their subsequent fungicide spray schedules until the end of the season. Without higher temperatures, it is challenging to distinguish between these alterations. As part of the Western Weather Work Group (, we are collaborating with public and private scientists to help control disease in the western US by providing access to real time, virtual and forecasted weather data. We have completed 2 years of field data, and we are adjusting how the GT index accounts for observed delay in fungal growth. These modifications will ultimately be used to assist both public and private end users of the GT model.